1. Field of the Invention
The present invention relates to a high-frequency power divider and combiner, and more specifically to a circuit for dividing and combining high-frequency power when a plurality of power amplifiers are parallel-operated.
2. Description of the Related Art
As power dividing and combining circuits for parallel-operating power amplifiers and combining together outputs produced from the power amplifiers so as to obtain large power, there have heretofore been used a 3 dB coupler type power divider and combiner, a Wilkinson's power divider and combiner, and an impedance transformer type power divider and combiner.
In a power amplifying apparatus of a type wherein a plurality of power amplifiers are parallel-operated to generate output power, which is then combined so as to produce large power, allowable output power is often controlled by changing the number of the power amplifiers. A description will now be made, as an illustrative example, of a case in which the input and output impedance in circuits are set to 50.OMEGA.,and wherein two, three or four power amplifiers are parallel-operated to thereby control allowable output power of a power amplifying apparatus.
When the 3 dB coupler type power divider and combiner is used, the power can be efficiently divided and combined only when the number of power amplifiers to be parallel-operated is 2.sup.n. On the other hand, when the Wilkinson's power divider and combiner is used, the power can be efficiently divided and combined when the number of parallel operating power amplifiers is an even number. In either case where the number of power amplifiers to be parallel-operated is four, the power amplifiers can be efficiently parallel-operated by using a 4-way divider and a 4-way combiner. However, when the number of the power amplifiers is reduced from four to three or two, the power is used up by resistances as dummy loads for absorbing unbalanced power, which are used to obtain isolation between terminals of dividers and between those of combiners. Therefore, a power loss of about 2.5 dB is developed when the three power amplifiers are used, whereas a power loss of about 6.0 dB is developed when the two power amplifiers are used.
In order to efficiently divide the power and combine it in 2-way combination form, it is necessary to replace the divider and the combiner with a 2-way divider and a 2-way combiner respectively. When the divider and the combiner are replaced by others respectively, the operation of a power amplifying apparatus should be temporarily stopped. Therefore, the 3 dB coupler type power divider and combiner and the Wilkinson's power divider and combiner are not suited to a method of making the change in the number of the units of the power amplifiers to thereby adjust or control the allowable output power of the apparatus.
Accordingly, an impedance transformer type 4-way divider and 4-way combiner will be shown in FIG. 6(A) as one example. In FIG. 6(A), D indicates a power dividing circuit (hereinafter called merely a "divider") and S indicates a power combining circuit (hereinafter called merely a "combiner"). In addition, A.sub.1 through A.sub.4 indicate power amplifiers respectively. However, the impedance transformer type divider and combiner, different from the 3 dB coupler type power divider and combiner and the Wilkinson's power divider and combiner, provide no resistance as a dummy load for unbalanced power absorption, but only an impedance matching function is provided and hence no isolation is obtained between terminals. Therefore, the isolation between adjacent output terminals for outputs 1 through 4 of the divider and the isolation between adjacent input terminals for inputs 1 through 4 of the combiner are obtained from either isolators or circulators provided at inputs and outputs of respective power amplifiers Ai as shown in FIG. 6(B).
The combiner S and the divider D are identical in principle to each other. A description will therefore be made of the combiner S as an illustrative example. Transmission lines Ws.sub.1 through Ws.sub.4 each having an impedance of 50.OMEGA. and a line length of n.lambda./2 (where n=positive integer and .lambda.=wavelength of a used frequency) are provided between the respective input terminals of the combiner S and a combining point P thereof. Since the lines of 50.OMEGA. impedance are 4-way combined, the impedance at the combining point P is brought to 12.5.OMEGA. (=50 .OMEGA./4).
An impedance transformer is used to convert or transform the impedance at the combining point P into output impedance of 50.OMEGA.. For example, the impedance transformer is composed of a transmission line Ws.sub.5 having the impedance of 25.OMEGA. and a line length of .lambda./4. Incidentally, the divider D is also constructed in a manner similar to the combiner S. In this case, transmission lines Wd.sub.1 through Wd.sub.4 correspond to the transmission lines Ws.sub.1 through Ws.sub.4 and a transmission line Wd.sub.5 serving as the impedance transformer corresponds to the transmission line Ws.sub.5. By setting the combiner S and the divider D in this way, a division loss is 0 db whereas a combination loss is 0 dB, which result in the total loss of 0 dB.
A description will now be made of a case in which the control of allowable output power is carried out by making a change in the number of power amplifiers. As shown in FIG. 7, one of input terminals of a 4-way combiner S is opened and three power amplifiers are connected in parallel to the corresponding input terminals so as to produce the output of the 4-way combiner in the form of 3-combination. The length of each of transmission lines, which extend from the power amplifiers A.sub.1, A.sub.2, A.sub.3 to a combining point P, is n.lambda./2. Thus, the impedance as seen in the direction of the opened input terminal of the 4-way combiner from the combining point P is equivalently brought to infinity. Therefore, the impedance at the combining point P reaches about 16.7.OMEGA. (=50 .OMEGA./3).
Further, an impedance transformer, which extends from the combining point P to the output terminal, is constructed so as to transform 12.5.OMEGA. into 50 .OMEGA.. Thus, when this impedance transformer is used so as to transform 16.7.OMEGA. into 50.OMEGA., the reflection coefficient produced due to impedance mismatching is determined by the following equation: ##EQU1##
The reflection loss produced due to the impedance mismatching is determined by the following equation: ##EQU2## Thus, the reflection loss is brought to about 0.09 dB.
The divider D also gives rise to the reflection loss in a manner similar to the combiner S. Therefore, the reflection loss is doubled over the entire device, that is, the reflection loss amounts to about 0.18 dB.
When the combiner S is used as a 2-combiner in the same manner as described above, the impedance at a combining point P is brought to 25.OMEGA. (=50 .OMEGA./2) as shown in FIG. 8. Therefore, the loss produced due to impedance mismatching of an impedance transformer is brought to about 0.5 dB. Thus, the loss in the combiner S and the divider D reaches about 1.0 dB in total.
Even when the 4-way combiner is used as a 3-way combiner or a 2-way combiner as described above, the power combination can be made although only the mismatch loss due to the impedance transformer is produced. The input power division can also be carried out in a manner similar to the power combination. However, a division loss and a combination loss due to mismatching increase when the number of divisions and the number of combinations are reduced.